Teats & Nipples
Teat ( in UK English) and Nipple (in US English) are essentially same. They are made of silicone rubber in the shape of mothers nipple, though are generally designed to be slimmer than the mother's nipple. Specialized Nipples are marketed that attempt to mimic the shape of the breast to help babies to switch back and forth between bottle feeding and breastfeeding without causing 'nipple confusion'.
Nipples also come in a selection of flow rates, based on the age of the infant. Different flow rate nipple either has more holes or larger holes. Variable flow rate nipple is available for older infants. Asymmetric hole in the nipple cause variable flow by turning the bottle/ nipple.